Resistor load differential amplifiers that include a load resistor, a differential transistor pair and a tail current source are known as an example of amplifiers used for high-speed data communications. The operation point of a resistor load differential amplifier is controlled by using the bias circuit. However, when an amplifier is formed on an integrated circuit, the characteristics of the circuit elements, such as transistors, resistors, etc., included in the amplifier may have variations. Thus, it is demanded that the bias circuit can control the operation point of an amplifier appropriately even when the characteristics of the circuit elements on the integrated circuit have variations.
FIG. 1 illustrates an example of an amplifier circuit that amplifies a differential signal. Note that the amplifier illustrated in FIG. 1 is a resistor load differential amplifier, and includes load resistors RL, differential transistor pair M1 and tail transistor M2.
The bias circuit generates a current that depends upon the resistance value of resistor Rref. In other words, the voltage of node N illustrated in FIG. 1 is maintained at Vref through negative feedback. In such a case, current Iref, which flows through resistor Rref, is Vref/Rref. When it is assumed that the current ratio between transistors M51 and M52 is 1:1, the current that flows through transistor M53 is Iref. Also, when it is assumed that the current ratio between transistors M53 and M2 is 1:2, the current that flows through transistor M2 is 2Iref. Accordingly, the current that flows through each of the transistors in differential transistor pair M1 is Iref. In such a case, operation point Vout of this amplifier is expressed by Formula 1. In the formula, Vdd represents a power supply voltage.
                                                                        V                out                            =                            ⁢                                                V                  dd                                -                                                      I                    ref                                    ⁢                                      R                    L                                                                                                                          =                            ⁢                                                V                  dd                                -                                                                            R                      L                                                              R                      ref                                                        ⁢                                      V                    ref                                                                                                          (        1        )            
As described above, operation point Vout of an amplifier is determined based on the ratio between resistor RL and resistor Rref. When the resistance value of for example resistor RL becomes higher than a target value because of manufacturing variations, the resistance value of resistor Rref also becomes higher at the same ratio. In other words, the ratio between resistor RL and resistor Rref does not depend upon manufacturing variations. Thus, the operation point of an amplifier can be configured without depending upon the manufacturing variations.
Note that differential amplifiers and circuits that control differential amplifiers are disclosed by for example Japanese Laid-open Patent Publication No. 2001-185964, Japanese Laid-open Patent Publication No. 2007-184688 and Japanese Laid-open Patent Publication No. 9-74338.
As described above, the operation point of an amplifier can be configured without depending upon the manufacturing variations. However, it is not easy to stably maximize the amplitudes of output signals of amplifiers by using the conventional techniques. For example, operation point Vout, which is expressed by Formula 2, is needed in order to maximize the amplitudes of output signals in the resistor load differential amplifier illustrated in FIG. 1. Vdd represents a power supply voltage. Vth1 represents the threshold voltage of each of the transistors in differential transistor pair M1. Vdsat2 represents the saturation drain voltage of tail transistor M2.
                              V          out                =                              V            dd                    -                                    1              2                        ⁢                          {                                                V                  dd                                -                                  (                                                            V                                              th                        ⁢                                                                                                  ⁢                        1                                                              +                                          V                                              dsat                        ⁢                                                                                                  ⁢                        2                                                                              )                                            }                                                          (        2        )            
As described above, the operation point that leads to the maximum amplitudes of output signals depends upon the characteristics of differential transistor pair M1 and tail transistor M2. Accordingly when the characteristics of transistors vary due to manufacturing variations, the conventional techniques, in which operation points are configured based on resistance ratios, fail to compensate for the variations in the characteristics of transistors. In other words, the conventional techniques sometimes fail to maximum the amplitudes of output signals of amplifiers.